A ball screw device including a screw shaft and a ball screw nut which is screwed with the screw shaft via balls is often used as an electric actuator or a damper. For example, Japanese Patent Publication No. 6-300106A, discloses an electric actuator incorporating a ball screw device wherein a rotor of a motor is connected to a ball screw nut, so that the ball screw nut is rotated to linearly move a screw shaft in an axial direction thereof.